1.Field of the Invention
The invention relates to a method for magnetic resonance imaging of a body placed in a stationary and substantially homogeneous main magnetic field, which method includes the application of an excitation radio-frequency pulse (RF-pulse) for excitation of nuclear dipole moments in at least a portion of the body, and the application of a plurality of refocusing RF-pulses following the excitation RF-pulse and of gradient magnetic fields for generating position dependent magnetic resonance signals in the excited portion. The applied RF-pulses form, for example, a CPMG-sequence (Carr-Purcell-Meiboom-Gill sequence) which generates multiple nuclear magnetic resonance echo signals (NMR-signals) following the refocusing RF-pulses. The invention also relates to an apparatus for magnetic resonance imaging using such a method.
2. Description of the Related Art
Such a method for imaging is known from EP-A 0 175 184, which corresponds to U.S. Pat. No. 4,818,940. As indicated in that document, a selection of a portion of the body is made by the application of a gradient magnetic field during the application of the excitation RF-pulse. This results in the excitation of a slice of the body in which slice the Larmor frequency of a selected nucleus type in the magnetic field corresponds to the frequency of the RF-pulse. The slice selection gradient magnetic field is also applied during application of the refocusing RF-pulses. For position determination of the magnetic resonance signals, a gradient magnetic field with the gradient in a first direction within the slice is applied in the interval between the refocusing RF-pulses and the measurements of the nuclear magnetic resonance (NMR) signals for phase encoding of the NMR-signals. During the measurement a second gradient magnetic field, with its gradient in a second direction within the slice and perpendicular to the first direction provides frequency encoding of the NMR-signals.
In a perfectly executed sequence the time-integrated values of the gradients are equal between refocusing RF-pulses and are twice as large as the time-integrated value of the gradients in between the excitation RF-pulse and the first refocusing RF-pulse. To obtain this in practice may be difficult due to delays or other timing inaccuracies, and due to the occurrence of eddy currents caused by switching on and off the gradient magnetic fields. In particular the eddy currents cause the gradient magnetic field pulses to have tails that may extend further than the following RF-pulse. Consequently, the time-integrated values are disturbed which leads to undesirable deviations in the phases and angles to which the nuclear dipole moments are set and the image resulting after transformation of the measured NMR-signals will exhibit artefacts. Artefacts are local low contrast, local absence of an image or the presence of ghosts in the image.
A second source of similar imaging errors or artefacts resides in the refocusing RF-pulses. Due to the variation of the magnetic field strength in the selected slice, a direct consequence of the presence of the slice selection gradient magnetic field, the refocusing RF-pulse will have a certain variation in the flip-angle across the slice. Also, this effect results in different dipole moments feeling different electromagnetic fields where they should have been subject to the same field.
In case a volume is selected as the excited portion of the body, a slice selection gradient may be absent during the RF-pulses. In addition to first and second gradient magnetic fields, a third gradient magnetic field with its gradient perpendicular to the first and second gradient magnetic fields is applied in between the RF-pulses and the measurements of the NMR-signals for additional phase encoding in that direction. Also, the switching of this third gradient magnetic field may contribute to artefacts in the resulting image.